It may be nickname with a horribly embarrassing story behind it, or something family members or lovers use only when they're alone (in which case it overlaps with Affectionate Nickname). Often describes a character's appearance, lack of ability in one area or another, though sexual prowess (or endowment) is a common one. Can be a Berserk Button.
This isn't necessarily always played for comedy; [...] [tvtropes.org]